1. Field of the Invention
The present invention relates generally to fuel assemblies for nuclear reactors and, more particularly, is concerned with improved features for removably attaching the top nozzle on the guide thimbles of a fuel assembly for facilitating reconstitution thereof.
2. Description of the Prior Art
Conventional designs of fuel assemblies include a multiplicity of fuel rods held in an organized array by grids spaced along the fuel assembly length. The grids are attached to a plurality of control rod guide thimbles. Top and bottom nozzles on opposite ends of the fuel assembly are secured to the control rod guide thimbles which extend above and below the opposite ends of the fuel rods. At the top end of the fuel assembly, the guide thimbles are attached in openings provided in the top nozzle. Conventional fuel assemblies also have employed a fuel assembly hold-down device to prevent the force of the upward coolant flow from lifting a fuel assembly into damaging contact with the upper core support plate of the reactor, while allowing for changes in fuel assembly length due to core induced thermal expansion and the like. Such hold-down devices have included the use of springs surrounding the guide thimbles, such as seen in U.S. Pat. No. 3,770,583 (U.S. Pat. No. Re. 31,583) and U.S. Pat. No. 3,814,667 to Klumb et al. and U.S. Pat. No. 4,269,661 to Kmonk et al.
During operation of such assembly in a nuclear reactor, the fuel rods may occasionally develop cracks along their length resulting primarily from internal stresses, thus establishing the possibility that fission products having radioactive characteristics may seep or otherwise pass into the primary coolant of the reactor. In view of the high costs associated with replacing fuel assemblies containing failed fuel rods, both domestic and foreign utilities have indicated an interest in reconstitutable fuel assemblies in order to minimize their operating and maintenance expenses.
Conventional reconstitutable fuel assemblies incorporate design features arranged to permit the removal of individual failed fuel rods, the option to replace rods, followed by the additional use in the reactor and/or normal handling and storage of the affected fuel assembly. Reconstitution has been made possible by providing a fuel assembly with a removable top nozzle. The top nozzle is mechanically fastened usually by a threaded arrangement to the upper end of each control rod guide thimble assembly, and the top nozzle can be removed remotely from an irradiated fuel assembly while it is still submerged in neutron-absorbing liquid. With rod removal/replacement and after the top nozzle has been remounted on the control rod guide thimbles, the reconstituted assembly can then be reinserted into the reactor and used until the end of its useful life, and/or stored in spent fuel pools or other places in a safe, normal manner.
The above-cross referenced patent applications describe and illustrate reconstitutable fuel assemblies having different arrangements for removably attaching the top nozzle to the upper ends of the guide thimbles. When given the task to modify a preexisting fuel assembly wherein the top nozzle is not readily removable, prior top nozzle removable attachment arrangements such as those of the cross-referenced applications are instructive of possible approaches, but do not necessarily point toward the direction one should take to make the required modification in the simplest and least costly way. Consequently, a need exists for a fresh approach tailored to the particular preexiting top nozzle attachment structure, one which will achieve removability of the top nozzle with the minimum change in the design of preexisting parts.